The present invention relates to an apparatus and methods for fast two and three dimensional ultrasound image acquisition, using a transducer design with cross geometry of a phased array placement which reduces the level of side lobe amplitudes and extends the useful frequency range of the transducers.
A problem of an acquisition of three-dimensional images is very important and allows one to increase the diagnostic ability of acoustic scanners. Very strong steps and progress were done in this direction by the use of transducers that mechanical steered the acoustic beam. It is seen very clear how strong these beautiful three-dimensional images increase the ability of diagnosis and treatment of patients. However, these are non-real time images.
Acquisition of real time three-dimensional images requires new fast methods of acquisition of acoustic images. Such methods are described in U.S. Pat. No. 5,797,845 of Barabash et al. This method employs an ability of a linear array with small individual elements to produce a flat acoustic beam or a flat receive aperture steered into some angle range. A cross placement of two linear arrays allows one to obtain a pencil acoustic beam by cross-section of the flat acoustic beam and the flat receive aperture, and an ability to steer the pencil acoustic beam into some solid angle limited by a radiation directivity of array individual elements. A creation of synthetic receive apertures by the use of digital beam-formation as shown in former schematics, allows a realization of a fast method of acquisition of three dimensional images by a fast steering of the flat acoustic apertures along the coordinate parallel to the direction of the receive array.
From all schemes of cross transducers suggested in this patent a scheme of a cross transducer with one transmit and one receive array is most attractive. In addition to properties described above, the use of this simple scheme significantly reduces the apparatus volume.
However, the shaping of a beam with a low level of side lobe amplitudes is a problem for this simple scheme. Both ways, first a transmission of acoustic waves and second a reception of echo signals participate in the process of side lobe amplitude reduction. But, the efficiency of the second way is not as high as the usual transducers used for two dimensional image acquisition, when both transmit flat beams and flat receive apertures produced by array individual elements are focused along the same plane.
The same method and schematics of a cross transducer are described in the U.S. Pat. No: 5,901,708 of Chang et al. Additionally specified, is the presence of a shared central element and a pitch of an array individual element equal to xcex/2 of carrier frequency.
A problem of the shared element exists for a cross transducer with an odd number of transmit and receive array individual elements, and is absent when transmit and receive arrays have an even number of individual elements.
Another severe limitation of the use of this suggestion is specification of the pitch of array individual elements by xcex/2 steps. It means that for any frequency which can be used by an acoustic scanner, we must have a separate transducer with the pitch of xcex/2 for this frequency. For example, a most popular frequency range (2-10) MHz (2 MHz, xcex/2=0.385 mm; 4 MHz, xcex/2=0.1925 mm; 6 MHz, xcex/2=0.1283 mm; 8 MHz, xcex/2=0.09625 mm; 10 MHz, xcex/2=0.077 mm) usually covered by two transducers approximately must be covered by five transducers. It is necessary to point out here that the most expensive parts of any acoustic machine are the transducers. Every transducer has its own frequency range defined by a mechanical and an electrical design. Many patents and published articles are dedicated to efforts that extend the frequency range of transducers; the main reason being is to reduce the price of acoustic machines and to increasing the number of transducers is more costly.
Specification for the pitch of arrays of xcex/2 in acoustic technology is historically based on radar and sonar technologies. But, let us remember the main conditions when this rule is applicable:
Placement of array elements with the pitch P is equal to:
P=xcex(n+xc2xd)
where xcex is the wave length for the carrier frequency and n=0, 1, 2, 3, . . .
The lens is flat and is used for the shaping of a parallel beam. A difference in delays between adjacent individual elements is equal to 0 and a phase shift between individual elements of the array provided by the specified placement is xcfx80, 3xcfx80, 5xcfx80, . . .
The duration xcfx84tr of the transmitted pulse is long:
xcfx84tr greater than  greater than T,
where T is a period of the carrier frequency and the usual requirement is xcfx84tr greater than 100T.
An amplitude of side lobes in the direction normal to the beam has a resonance behavior and is minimal when the pitch of array individual elements is xcex(n+xc2xd) of the carrier frequency.
However, all these conditions are violated when the acoustic beam is shaped. Wave packages transmitted by transmit array individual elements are short to provide good space resolution; the beam shaped by the array has a definite focus; lenses which are used for the focusing of the acoustic beam are curved. It is impossible to satisfy the main condition (phase shift between adjacent array individual elements is xcfx80, 3xcfx80, 5xcfx80, . . . ) which provides a destructive superposition of waves and a low level of side lobe amplitudes, because delays of array individual elements are changed from one element to another and a difference between delays of adjacent elements is not equal to 0, especially for large beam scan angles.
An additional effect, which influences the position and amplitude of the side lobe, is the spherical shape of the wave packages emitted by a separate array individual element. They are crossed in the focus and provide the shaping of the main lobe. But the shape of wave packages have noticeable curvature for a reasonable value of F-numbers. Therefore, all array elements participate in the shaping of the main lobe and low level of side lobe amplitudes in the vicinity of the main lobe. The number of array elements along a scan line that participate in the shaping of the side lobe level is decreased for azimuth angles far away from the focus, and the level of side lobes begins to grow. The width of the zone when all array elements provides the shaping of the low level of side lobes near the main lobe depends on the duration of the pulse emitted by transducer elements.
Thus, it is the intent of the present invention to show schemes of a simple cross transducer with one transmit and one receive array which has no problem of a shared central element.
Another intent of the present invention is to show versions of cross transducers with a shared central element, but the design of the central crossing area allows one to separate transmit and receive returns and increase the signal-to-noise ratio.
We would like to present a method of the use of cross transducers for acquisition of real time two and three dimensional images, which allows an extension of a frequency range and an efficient use of cross transducers and the increase of the range of beam scan angles.
In particular, this method includes the next steps:
An optimal choice of the pitch of transmit and receive array individual elements to provide an extension of the dynamic range of frequency for the cross transducer.
An optimal choice of the duration of transmitted pulses used for irradiation of an investigated object providing an increase of the dynamic range of beam scan angles.
An optimal processing of digitized echo signal amplitudes to provide a correction of amplitudes of echo signals depending on the scan angle to reduce an influence of the directivity of the array individual elements radiation and provide a uniform brightness of images.
An optimal processing of digitized echo signal amplitudes to provide a reduction of side lobe amplitudes based on a knowledge of a behavior of side lobe amplitudes depending on the position of voxels used for a creation of three dimensional images in the investigated space. The reduction of side lobe amplitudes can be done by the correction of side lobe amplitudes. A correction function can be defined by a simulation or measurements of side lobe amplitudes with a consequent normalization and deduction of them by an iteration process from amplitudes of other voxels placed on the same spherical surface.
Another method of a reduction of side lobe amplitudes for two dimensional image acquisition is presented. The sense of this method is an asymmetry in the position of array individual elements when they are grouped in three groups: two groups have positive and negative shifts along the lateral coordinate and one group non-shifted elements. Non-shifted elements can be used as transmit array elements (or receive elements) and groups of shifted elements can organize array for the reception (or transmission) of echo signals. Such an asymmetry provides an additional phase shift of transmit packages or received echo signals for area far away from the main lobe and reduces the level of side lobe amplitudes significantly.
The same effect of a suppression of the side lobe amplitudes is recognized for the simple cross transducer with one transmit and one receive arrays when they are shifted relatively each other for a distance is equal to xc2xc of the pitch.
The extension of the frequency range and the range of the beam scan angles allows the increase of the transducer field of view. An approach to the creation of the two and three dimensional images and the image format is described also.